Seeing as it makes the weapon slightly lubricated and stops snagging on other weapons so much (more so than talcum powder) It immensely decreases the chance of ripping if you accidentally snag your weapon on a tree or a wall. Not to mention silicone spray won't come off in rain and provides a larger amount of waterproofing, Seeing as talcum powder just washes off in the rain. You can get large bottles of it really cheaply (£6 ($9.68) for 1 liter) which is the same price the larp stores charge for the 60ml stuff. Also if i may add, as a larp weapon manufacturer I've noticed after a while talcum powder dries the latex out, whereas silicone spray keeps the latex healthy. We have this running saying in the workshop for stripping latex "it silicon-ed it will slip off like a nice tight sock, If it's talced you're in for the long run"

So simply yes, yes it's very good to silicone your weapon, it increases the life span by around 2 years for the latex.

In general, silicon based products of this kind are used as lubricants and sealants. I would imagine, in your case, that it would water proof the prop and make removal of other kinds of debris easier - similar to camping equipment. It may also help prevent minor abrasions - because it's lubricated.

Is it worth the money? That's something only you can decide. How valuable is that sword?